Competing magnetic orders in the superconducting state of heavy-fermion CeRhIn5.

Applied pressure drives the heavy-fermion antiferromagnet CeRhIn5 toward a quantum critical point that becomes hidden by a dome of unconventional superconductivity. Magnetic fields suppress this superconducting dome, unveiling the quantum phase transition of local character. Here, we show that [Formula: see text] magnetic substitution at the Ce site in CeRhIn5, either by Nd or Gd, induces a zero-field magnetic instability inside the superconducting state. This magnetic state not only should have a different ordering vector than the high-field local-moment magnetic state, but it also competes with the latter, suggesting that a spin-density-wave phase is stabilized in zero field by Nd and Gd impurities, similarly to the case of Ce0.95Nd0.05CoIn5 Supported by model calculations, we attribute this spin-density wave instability to a magnetic-impurity-driven condensation of the spin excitons that form inside the unconventional superconducting state.

Quantum Oscillations in Flux-Grown SmB_{6} with Embedded Aluminum.

SmB_{6} is a candidate topological Kondo insulator that displays surface conduction at low temperatures. Here, we perform torque magnetization measurements as a means to detect de Haas-van Alphen (dHvA) oscillations in SmB_{6} crystals grown by aluminum flux. We find that dHvA oscillations occur in single crystals containing embedded aluminum, originating from the flux used to synthesize SmB_{6}. Measurements on a sample with multiple, unconnected aluminum inclusions show that aluminum crystallizes in a preferred orientation within the SmB_{6} cubic lattice. The presence of aluminum is confirmed through bulk susceptibility measurements, but does not show a signature in transport measurements. We discuss the ramifications of our results.

Evidence of a Nodal Line in the Superconducting Gap Symmetry of Noncentrosymmetric ThCoC_{2}.

The newly discovered noncentrosymmetric superconductor ThCoC_{2} exhibits numerous types of unconventional behavior in the field dependent heat capacity data. Here we present the first measurement of the gap symmetry of ThCoC_{2} by muon spin rotation and relaxation (µSR) measurements. The temperature dependence of the magnetic penetration depth measured using the transverse field µSR experiment reveals the evidence of a nodal pairing symmetry. To understand this finding, we carry out calculations of the superconducting pairing eigenvalue and eigenfunction (pairing symmetry) due to the spin-fluctuation mechanism by directly implementing the ab initio band structures. We find that the system possesses a single Fermi surface with considerable three dimensionality and a strong nesting along the k_{z} direction. Such nesting promotes a superconducting state with a cosk_{z}-like pairing symmetry with a prominent nodal line on the k_{z}=±π/2 plane. The result agrees well with the experimental data.

Surface-dominated conduction up to 240 K in the Kondo insulator SmB6 under strain.

SmB6 is a strongly correlated mixed-valence Kondo insulator with a newly discovered surface state, proposed to be of non-trivial topological origin. However, the surface state dominates electrical conduction only below T∗ ≈ 4 K (ref. ), limiting its scientific investigation and device application. Here, we report the enhancement of T∗ in SmB6 under the application of tensile strain. With 0.7% tensile strain we report surface-dominated conduction at up to a temperature of 240 K, persisting even after the strain has been removed. This can be explained in the framework of strain-tuned temporal and spatial fluctuations of f-electron configurations, which might be generally applied to other mixed-valence materials. We note that this amount of strain can be induced in epitaxial SmB6 films via substrate in potential device applications.

4f Crystal Field Ground State of the Strongly Correlated Topological Insulator SmB_{6}.

We investigated the crystal-electric field ground state of the 4f manifold in the strongly correlated topological insulator SmB_{6} using core-level nonresonant inelastic x-ray scattering. The directional dependence of the scattering function that arises from higher multipole transitions establishes unambiguously that the Γ_{8} quartet state of the Sm f^{5} J=5/2 configuration governs the ground-state symmetry and, hence, the topological properties of SmB_{6}. Our findings contradict the results of density functional calculations reported so far.

Topological surface state in the Kondo insulator samarium hexaboride.

Topological invariants of electron wavefunctions in condensed matter reveal many intriguing phenomena. A notable example is provided by topological insulators, which are characterized by an insulating bulk coexisting with a metallic boundary state. Although there has been intense interest in Bi-based topological insulators, their behaviour is complicated by the presence of a considerable residual bulk conductivity. Theories predict that the Kondo insulator system SmB6, which is known to undergo a transition from a Kondo lattice metal to a small-gap insulator state with decreasing temperature, could be a topological insulator. Although the insulating bulk and metallic surface separation has been demonstrated in recent transport measurements, these have not demonstrated the topologically protected nature of the metallic surface state. Here we report thickness-dependent transport measurements on doped SmB6, and show that magnetic and non-magnetic doping results in contrasting behaviour that supports the conclusion that SmB6 shows virtually no residual bulk conductivity.

Reemergent superconductivity and avoided quantum criticality in Cd-doped CeIrIn(5) under pressure.

We investigated the electrical resistivity and heat capacity of 1% Cd-doped CeIrIn_{5} under hydrostatic pressure up to 2.7 GPa, near where long-range antiferromagnetic order is suppressed and bulk superconductivity suddenly reemerges. The pressure-induced T_{c} is close to that of pristine CeIrIn_{5} at 2.7 GPa, and no signatures of a quantum critical point under pressure support a local origin of the antiferromagnetic moments in Cd-CeIrIn_{5} at ambient pressure. Similarities between superconductors CeIrIn_{5} and CeCoIn_{5} in response to Cd substitutions suggest a common magnetic mechanism.

Non-Kondo-like electronic structure in the correlated rare-earth hexaboride YbB(6).

We present angle-resolved photoemission studies on the rare-earth-hexaboride YbB(6), which has recently been predicted to be a topological Kondo insulator. Our data do not agree with the prediction and instead show that YbB(6) exhibits a novel topological insulator state in the absence of a Kondo mechanism. We find that the Fermi level electronic structure of YbB(6) has three 2D Dirac cone like surface states enclosing the Kramers's points, while the f orbital that would be relevant for the Kondo mechanism is ∼1 eV below the Fermi level. Our first-principles calculation shows that the topological state that we observe in YbB(6) is due to an inversion between Yb d and B p bands. These experimental and theoretical results provide a new approach for realizing novel correlated topological insulator states in rare-earth materials.

Mixed-valence state in the dilute-impurity regime of La-substituted SmB6.

Homogeneous mixed-valence (MV) behaviour is one of the most intriguing phenomena of f-electron systems. Despite extensive efforts, a fundamental aspect which remains unsettled is the experimental determination of the limiting cases for which MV emerges. Here we address this question for SmB6, a prototypical MV system characterized by two nearly-degenerate Sm2+ and Sm3+ configurations. By combining angle-resolved photoemission spectroscopy (ARPES) and x-ray absorption spectroscopy (XAS), we track the evolution of the mean Sm valence, vSm, in the SmxLa1-xB6 series. Upon substitution of Sm ions with trivalent La, we observe a linear decrease of valence fluctuations to an almost complete suppression at x = 0.2, with vSm ~ 2; surprisingly, by further reducing x, a re-entrant increase of vSm develops, approaching the value of vimp ~ 2.35 in the dilute-impurity limit. Such behaviour departs from a monotonic evolution of vSm across the whole series, as well as from the expectation of its convergence to an integer value for x → 0. Our ARPES and XAS results, complemented by a phenomenological model, demonstrate an unconventional evolution of the MV character in the SmxLa1-xB6 series, paving the way to further theoretical and experimental considerations on the concept of MV itself, and its influence on the macroscopic properties of rare-earth compounds in the dilute-to-intermediate impurity regime.

NMR study of in-plane twofold ordering in URu(2)Si(2).

We report (29)Si NMR spectra and Knight shift measurements as a function of applied field orientation in the (001) basal plane of URu(2)Si(2). Observed linewidth oscillations confirm the in-plane twofold ordered domain state observed in recent magnetic susceptibility measurements. Analysis of our linewidth data leads to estimate ∼ 0.4% for the twofold intrinsic (monodomain) susceptibility anisotropy in the basal plane, a value ∼ 15 times smaller than that obtained from recent susceptibility measurements.

Magnetism and superconductivity driven by identical 4f states in a heavy-fermion metal.

The apparently inimical relationship between magnetism and superconductivity has come under increasing scrutiny in a wide range of material classes, where the free energy landscape conspires to bring them in close proximity to each other. Particularly enigmatic is the case when these phases microscopically interpenetrate, though the manner in which this can be accomplished remains to be fully comprehended. Here, we present combined measurements of elastic neutron scattering, magnetotransport, and heat capacity on a prototypical heavy fermion system, in which antiferromagnetism and superconductivity are observed. Monitoring the response of these states to the presence of the other, as well as to external thermal and magnetic perturbations, points to the possibility that they emerge from different parts of the Fermi surface. Therefore, a single 4f state could be both localized and itinerant, thus accounting for the coexistence of magnetism and superconductivity.

Limit cycle and anomalous capacitance in the Kondo insulator SmB6.

We report a frequency coding limit cycle and anomalous capacitance in the Kondo insulator SmB6 at low temperatures where the insulating gap becomes fully opened. The limit cycle appears to be associated with local activity and autocatalytic temporal pattern formation, as occurs in biological systems. The measured anomalous capacitance may indicate surface and bulk separation, suggesting the formation of a surface conducting state. The biological analogy suggests lossless information transport and complex information coding, and the surface state with a superconductor would provide a possible venue for quantum computing resources without decoherence.

Electronic tuning and uniform superconductivity in CeCoIn5.

We report a globally reversible effect of electronic tuning on the magnetic phase diagram in CeCoIn(5) driven by electron (Pt and Sn) and hole (Cd, Hg) doping. Consequently, we are able to extract the superconducting pair breaking component for hole and electron dopants with pressure and codoping studies, respectively. We find that these nominally nonmagnetic dopants have a remarkably weak pair breaking effect for a d-wave superconductor. The pair breaking is weaker for hole dopants, which induce magnetic moments, than for electron dopants. Furthermore, both Pt and Sn doping have a similar effect on superconductivity despite being on different dopant sites, arguing against the notion that superconductivity lives predominantly in the CeIn(3) planes of these materials. In addition, we shed qualitative understanding on the doping dependence with density functional theory calculations.

Direct observation of the quantum critical point in heavy fermion CeRhSi3.

We report on muon spin rotation studies of the noncentrosymmetric heavy fermion antiferromagnet CeRhSi3. A drastic and monotonic suppression of the internal fields, at the lowest measured temperature, was observed upon an increase of external pressure. Our data suggest that the ordered moments are gradually quenched with increasing pressure, in a manner different from the pressure dependence of the Néel temperature. At 23.6 kbar, the ordered magnetic moments are fully suppressed via a second-order phase transition, and T(N) is zero. Thus, we directly observed the quantum critical point at 23.6 kbar hidden inside the superconducting phase of CeRhSi3.

Quantum critical Kondo quasiparticles probed by ESR in ß-YbAlB4.

Electron spin resonance (ESR) can probe conduction electrons (CE) and local moment (LM) spin systems in different materials. A CE spin resonance (CESR) is observed in metallic systems based on light elements or with enhanced Pauli susceptibility. LM ESR can be seen in compounds with paramagnetic ions and localized d or f electrons. Here we report a remarkable and unprecedented ESR signal in the heavy-fermion superconductor ß-YbAlB4 [S. Nakatsuji et al., Nature Phys. 4, 603 (2008)] which behaves as a CESR at high temperatures and acquires characteristics of the Yb³âº LM ESR at low temperature. This dual behavior strikes as an in situ unique observation of the Kondo quasiparticles in a quantum critical regime. The proximity to a quantum critical point may favor the appearance of this dual character of the ESR signal in ß-YbAlB4.

Magnetic field induced orbital polarization in cubic YbInNi4: determining the quartet ground state using x-ray linear dichroism.

We have been able to induce a linear dichroic signal in the Yb M(5) x-ray absorption white line of cubic YbInNi(4) by the application of a magnetic field. The nonzero integrated intensity of the magnetic field induced dichroic spectrum indicates a net noncubic 4f orbital polarization. A quantitative analysis of the temperature and field strength dependence establishes that the crystal-field ground state is a Γ(8) quartet. The results demonstrate the potential of magnetic field induced linear dichroism as a new powerful approach for the investigation of the degeneracy and orbital degrees of freedom of cubic heavy-fermion and Kondo systems.

Magnetoresistance from quantum interference effects in ferromagnets

The desire to maximize the sensitivity of read/write heads (and thus the information density) of magnetic storage devices has stimulated interest in the discovery and design of new magnetic materials exhibiting magnetoresistance. Recent discoveries include the 'colossal' magnetoresistance in the manganites and the enhanced magnetoresistance in low-carrier-density ferromagnets. An important feature of these systems is that the electrons involved in electrical conduction are different from those responsible for the magnetism. The latter are localized and act as scattering sites for the mobile electrons, and it is the field tuning of the scattering strength that ultimately gives rise to the observed magnetoresistance. Here we argue that magnetoresistance can arise by a different mechanism in certain ferromagnets--quantum interference effects rather than simple scattering. The ferromagnets in question are disordered, low-carrier-density magnets where the same electrons are responsible for both the magnetic properties and electrical conduction. The resulting magnetoresistance is positive (that is, the resistance increases in response to an applied magnetic field) and only weakly temperature-dependent below the Curie point.

High-temperature weak ferromagnetism in a low-density free-electron gas.

The magnetic properties of the ground state of a low-density free-electron gas in three dimensions have been the subject of theoretical speculation and controversy for seven decades. Not only is this a difficult theoretical problem to solve, it is also a problem which has not hitherto been directly addressed experimentally. Here we report measurements on electron-doped calcium hexaboride (CaB6) which, we argue, show that-at a density of 7× 1019 electrons cm-3-the ground state is ferromagnetically polarized with a saturation moment of 0.07 µB per electron. Surprisingly, the magnetic ordering temperature of this itinerant ferromagnet is 600 K, of the order of the Fermi temperature of the electron gas.

Rare-Earth elements and high pressures.

Praseodymium, under very high pressures, shows a magnetic behavior similar to that of cerium at normal pressure.

Heavy-electron metals: new highly correlated States of matter.

Heavy-electron metals exhibit highly correlated electronic behavior at liquid helium temperatures, with conduction-electron masses some hundred times the free-electron mass. Whether "normal," antiferromagnetic, or superconducting, their electronic behavior differs drastically from their ordinary metallic counterparts. The physical origin of the large mass and unusual superconducting and magnetic properties is the strong coupling between the conduction electrons and the local f-electron moment fluctuations characteristic of these materials.